(1) Field of the Invention
The present invention relates to the field of axial-flow turbines and, more particularly, those of the type in which a gas containing dust and mist such as blast-furnace gas is utilized as the working fluid, and it contemplates to provide an improved axial-flow turbine, in which fouling with the dust contained in the gas can be effectively prevented and which can be operated at a high efficiency.
(2) Description of the Prior Art
Furnace gas has a high pressure value as well as a high temperature. In addition, it is generated in great volumes, and to discharge it into air as it is emitted from the furnace directly means loss of great amounts of energy. Thus, it has been proposed to recover the high pressure energy possessed by furnace gas, as for example in the U.S. Pat. No. 3,818,707 to Boundard et al. This patent proposes, as a means for removing dust from the furnace gas, to charge the gas into a wet-type gas cleansing device or venturi scrubber, in which the gas is washed to reduce the dust content, and then feed the gas into the turbine.
For a power or energy recovery apparatus of this type, centrifugal turbines are generally employed: Centrifugal turbine have fundamental structural features such that they hardly permit dust clogging to occur, have a relatively high resistivity against impurities in gas such as dust, and allow the dust to be discharged with relative ease.
However, the energy recovery apparatus relied on a centrifugal turbine has to be relatively great in size and expensive to install; in addition, the efficiency of the turbine itself is relatively low and rate of energy recovery cannot be disirably high.
In comparison to centrifugal turbines, axial-flow turbines have an essentially desirable characteristic such that they can be smaller in size and yet have a higher efficiency. However, the axial-flow turbine, too, have certain difficulties. For example, they permit dust to attach to their blades with relative ease, whereby it rather easily tends to occur that their efficiency is lowered, that the flow rate of gas is affected when the nozzles are clogged, if partly, with the dust, or that abnormal vibration is generated due to dust accumulation on the rotor blades. Dust attached to blades undergoes agglomeration, and when the dust agglomerate has grown to a certain size, it suddenly becomes liberated from the blade surface and sent into the succeeding stage or stages, whereby it is likely to impair or break fixed blade or mobile blade driven at a high speed.
In U.S. Pat. No. 3,818,707 above referred to, the saturation degree of the furnace gas to be charged into the turbine is enhanced by way of having the moisture in the gas evaporated, and while maintaining the saturation of gas at least at the same as the elevated degree, gas is expanded in the turbine. However, to adjust the degree of saturation of gas as prescribed not only requires a highly attentive care to exert but also make it extremely difficult in case of failure to effect an exact adjustment to check the attachment of dust in the turbine.
The manner in which dust becomes attached to the mobile blades and so forth can vary depending on a difference in the amount of dust in the gas, flow rate of gas and so forth. For example, in the instance of a blast furnace, the exhaust gas that has been treated through a venturi scrubber, a wet-type dust removing device, normally contains dust in an amount on the order of 100 mg/Nm.sup.3 and moisture 3-5 g/Nm.sup.3. Such gas very easily tends to attach about wall surfaces of its path in a turbine, particularly the surfaces of nozzles or fixed blades of an upstream stage or stages.
In order to prevent such dust attachment or dust fouling from occurring, the present inventors have attempted to jet water onto portions of the furnace which easily permit dust fouling to take place, but good results were not obtained.
In this connection, there also has been a proposed method, according to which before it is blasted into the turbine, a gas which has been suitably cleansed is subjected to a heat application to completely dry it. However, to put such a method into operation indispensably requires a particular device for partly burning the furnace gas or a heat exchanger, whereby not only is the cost of installation increased but also the operating conditions become more complicated.